Method and system for programming and driving active matrix light emitting device pixel having a controllable supply voltage

• US 9,741,292 B2
• Filed: 09/02/2015
• Issued: 08/22/2017
• Est. Priority Date: 12/07/2004
• Status: Active Grant

• Current Assignee: IGNIS Innovation Incorporated
• Original Assignee: IGNIS Innovation Incorporated
• Inventors: Nathan, Arokia, Chaji, Gholamreza, Servati, Peyman
• Primary Examiner(s): Pham, Long D
• Application Number:
  US14/843,211
• Publication Number:
  US 20150379932A1
• Time in Patent Office: 720 Days
• Field of Search: 345 82
• CPC Class Codes: G09G 2300/0465 Improved aperture ratio, e.g. by size reduction of the pixel circuit, e.g. for improving the pixel density or the maximum displayable luminance or brightness
• CPC Class Codes: G09G 2300/0842 forming a memory circuit, e.g. a dynamic memory with one capacitor
• CPC Class Codes: G09G 2300/0852 being a dynamic memory with more than one capacitor
• CPC Class Codes: G09G 2310/0262 The addressing of the pixel, in a display other than an active matrix LCD, involving the control of two or more scan electrodes or two or more data electrodes, e.g. pixel voltage dependent on signals of two data electrodes
• CPC Class Codes: G09G 2310/06 Details of flat display driving waveforms
• CPC Class Codes: G09G 2310/061 for resetting or blanking
• CPC Class Codes: G09G 2320/043 Preventing or counteracting the effects of ageing
• CPC Class Codes: G09G 3/3233 with pixel circuitry controlling the current through the light-emitting element
• CPC Class Codes: G09G 3/3258 with pixel circuitry controlling the voltage across the light-emitting element
• CPC Class Codes: G09G 3/3696 Generation of voltages supplied to electrode drivers

Abstract

Method and system for programming and driving active matrix light emitting device pixel is provided. The pixel is a voltage programmed pixel circuit, and has a light emitting device, a driving transistor and a storage capacitor. The pixel has a programming cycle having a plurality of operating cycles, and a driving cycle. During the programming cycle, the voltage of the connection between the OLED and the driving transistor is controlled so that the desired gate-source voltage of a driving transistor is stored in a storage capacitor.

Claims

1. A method of programming a pixel circuit that drives a current-driven organic light emitting device independent of a threshold voltage of a drive transistor connected in series to the organic light emitting device, the pixel circuit further including a first switch transistor connected between the organic light emitting device and a node of the pixel circuit, the method comprising:

adjusting during a programming cycle a controllable voltage supply to a compensation voltage;

in a first operating cycle of the programming cycle, selecting a select line, to turn on a second switch transistor coupled to the drive transistor, for applying a reference voltage to the drive transistor through the second switch transistor, and to turn on the first switch transistor, and applying to a data line connected to the second switch transistor a voltage that exceeds a programming voltage to be applied to the drive transistor;

responsive to the adjusting, allowing the node of the pixel circuit to charge or discharge through the drive transistor, via the first switch transistor, until the drive transistor turns off, thereby establishing the threshold voltage of the drive transistor across the drive transistor;

in a further operating cycle of the programming cycle following the first operating cycle, selecting the select line to turn on the second switch transistor for applying a programming voltage from the data line to the drive transistor through the second switch transistor, thereby establishing a fixed voltage applied to the drive transistor according to both the threshold voltage and the applied programming voltage, wherein the programming voltage is lower than the reference voltage;

responsive to the applying the programming voltage, selecting the select line with an intermediate selection voltage being sufficient to turn on the second switch transistor while turning off the first switch transistor; and

applying a second reference voltage, via the data line, to allow the fixed voltage in the pixel circuit to be independent of the reference voltage.

2. The method of claim 1, further comprising:

setting the controllable voltage supply to an operating voltage; and

deselecting the select line to complete the programming cycle and initiate a drive cycle during which the light emitting device is turned on according to the programming voltage while maintaining the fixed voltage on the drive transistor.

3. The method of claim 2, wherein the compensation voltage is sufficient to prevent the organic light emitting device from being turned on prior to the initiation of the drive cycle.

4. The method of claim 1, wherein the pixel circuit includes a storage capacitor coupled to the gate terminal of the drive transistor, and wherein the node of the pixel circuit is coupled to the gate terminal of the transistor such that the allowing the node of the pixel circuit to discharge is carried out by charging the storage capacitor through the first switch transistor and the drive transistor.

5. The method of claim 1, wherein a voltage stored on the node following the drive transistor turning off is different from the voltage on a voltage supply line connected to a terminal of the drive transistor opposite the organic light emitting device by the threshold voltage of the drive transistor.

6. The method of claim 1, wherein the select line remains selected during the allowing the node to charge or discharge and the applying the programming voltage.

7. The method of claim 1, wherein the pixel circuit includes a storage capacitor coupled between the second switch transistor and the node of the pixel circuit for being charged with the fixed voltage, and a first capacitor coupled between a voltage supply line connected to the drive transistor opposite the light emitting device and the storage capacitor, the second switch transistor connected to a node between the first capacitor and the storage capacitor, and wherein the applying the second reference voltage discharges the first capacitor and leaves the fixed voltage on the storage capacitor.

8. A display system comprising:

a pixel circuit including a drive transistor, a light emitting device, a first switch transistor, and a second switch transistor,

the drive transistor having a first terminal connected to the light-emitting device, the drive transistor having a threshold voltage that shifts during operation of the drive transistor,

the first switch transistor coupled between a data line and the second switch transistor, and

the second switch transistor having a first terminal connected to the first terminal of the drive transistor and a second terminal connected to a gate terminal of the drive transistor, such that, during a compensation cycle of a programming cycle while the second switch transistor is selected via a select line and while a high voltage, which exceeds a programming voltage to be applied to the drive transistor through the first switch transistor, is applied to the data line, the second switch transistor is allowed to charge or discharge a node of the pixel circuit, through the drive transistor, via the second switch transistor, until the drive transistor turns off, thereby establishing the threshold voltage of the drive transistor between the gate terminal and the first terminal of the drive transistor;

during an operating cycle of the programming cycle following the compensation cycle, via an intermediate voltage on the select line, turning off the second switch transistor and turning on the first switch transistor for applying the programming voltage from the data line to the drive transistor through the first switch transistor, thereby establishing a fixed voltage applied to the drive transistor according to both the threshold voltage and the applied programming voltage, wherein the programming voltage is lower than the high voltage; and

a controllable power supply connected to the light-emitting device for supplying the drive transistor with a voltage that is adjusted to a compensation voltage sufficient to turn off the light emitting device, during the compensation cycle.

9. The display system of claim 8, wherein the node that is charged or discharged, via the second switch transistor, is directly connected to the gate terminal of the drive transistor.

10. The display system of claim 8 in which the controllable voltage source maintains a substantially constant pixel current as the threshold voltage of the drive transistor changes with the aging of the drive transistor.

11. The display system of claim 8 in which the light-emitting device includes an OLED supplied with the stable pixel current from the drive transistor, and the stable pixel current maintains a substantially constant brightness of the light emitted by the OLED.

12. The display system of claim 8 in which the light-emitting device includes an OLED having a voltage V_OLED that increases as the OLED ages.

13. The display system of claim 8, wherein the pixel circuit is configured to receive, via the first switch transistor, a reference voltage applied to a terminal of a storage capacitor, the storage capacitor being coupled to the gate terminal of the drive transistor such that the storage capacitor is charged according to the threshold voltage of the drive transistor during the compensation cycle.

14. The display system of claim 8, wherein the pixel circuit further includes:

a storage capacitor coupled to the gate terminal of the drive transistor, the storage capacitor adapted to apply a voltage to the drive transistor during a driving cycle of the pixel circuit during which the pixel circuit is operated to drive the light emitting device to emit light according to programming information, wherein

the first switch transistor is adapted to selectively couple the data line to the storage capacitor during the compensation cycle and during the programming cycle of the pixel circuit during which the pixel circuit receives the programming voltage according to the programming information.

15. The display system of claim 8 wherein the light-emitting device is an organic light emitting diode and the drive transistor is an n-type or p-type thin film transistor.